350 HOW ELECTRICITY IS USED IN THE HOME 



dry cell. The button is a device for making and break- 

 ing the circuit. When you press the button you connect 

 the parts of the wire which at other times are separated 

 by an air space. Electricity can flow only when there is 

 a complete metal circuit from one pole of the battery 

 back to the other pole. The wire used must be insulated 

 to prevent leakage and short circuits, should the wire 

 happen to come in contact with metal or wet wood, or two 

 parts of the wire come together. 



Experiment. To demonstrate the electric bell. 



Materials: Electric bell. Electric cell. Wires. Push button. 

 Method: (A) Study the bell. Make out these parts : two binding posts, 



electromagnet, armature, spring, contact post, contact screw, hammer, 



gong. Trace the wire connection through the different parts from 



one binding post to the other. 

 Note: In some bells a part of the wire circuit is omitted, and the base 



of the bell carries the current from one part of the bell to one of the 



binding posts. Is that the case with the bell in hand? 



Connect bell, cell, and button in circuit. Adjust contact screw if 



necessary. Make the bell ring. 

 Explanation: Explain in detail just why the bell rings. Make diagram 



and trace, using arrows, the passage of the current from the cell back 



to the cell. 



How the bell works. The bell itself consists of the base 

 support, binding posts, electromagnet, armature, hammer, 

 spring, contact post, and gong. These parts are all shown 

 in the diagram. The wires from the battery and button 

 are connected to the two binding posts ; the current then 

 goes to the electromagnet, to the spring and armature, 

 to the contact post, and thence back to the other bind- 

 ing post. A strip of platinum on the armature makes 

 contact with a platinum point from the contact post. 

 These two pieces of platinum are held in position touching 

 each other by the spring when no current flows. 



